Hemodilution does not alter the coronary vasodilating effects of endogenous or exogenous nitric oxide

Unraveling the Gordian knot of coronary pressure-flow autoregulation

The coronary circulation has the inherent ability to maintain myocardial perfusion constant over a wide range of perfusion pressures. The phenomenon of pressure-flow autoregulation is crucial in response to flow-limiting atherosclerotic lesions which diminish coronary driving pressure and increase risk of myocardial ischemia and infarction. Despite well over half a century of devoted research, understanding of the mechanisms responsible for autoregulation remains one of the most fundamental and contested questions in the field today. The purpose of this review is to highlight current knowledge regarding the complex interrelationship between the pathways and mechanisms proposed to dictate the degree of coronary pressure-flow autoregulation. Our group recently likened the intertwined nature of the essential determinants of coronary flow control to the symbolically unsolvable "Gordian knot". To further efforts to unravel the autoregulatory "knot", we consider recent challenges to the local metabolic and myogenic hypotheses and the complicated dynamic structural and functional heterogeneity unique to the heart and coronary circulation. Additional consideration is given to interrogation of putative mediators, role of K+ and Ca2+ channels, and recent insights from computational modeling studies. Improved understanding of how specific vasoactive mediators, pathways, and underlying disease states influence coronary pressure-flow relations stands to significantly reduce morbidity and mortality for what remains the leading cause of death worldwide.

Disentangling the Gordian knot of local metabolic control of coronary blood flow

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o₂) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po₂ has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po₂ relative to MV̇o₂ are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Local metabolic hypothesis is not sufficient to explain coronary autoregulatory behavior

The local metabolic hypothesis proposes that myocardial oxygen tension determines the degree of autoregulation by increasing the production of vasodilator metabolites as perfusion pressure is reduced. Thus, normal physiologic levels of coronary venous PO₂, an index of myocardial oxygenation, are proposed to be required for effective autoregulation. The present study challenged this hypothesis through determination of coronary responses to changes in coronary perfusion pressure (CPP 140-40 mmHg) in open-chest swine in the absence (n = 7) and presence of euvolemic hemodilution (~ 50% reduction in hematocrit), with (n = 5) and without (n = 6) infusion of dobutamine to augment MVO₂. Coronary venous PO₂ decreased over similar ranges (~ 28-15 mmHg) as CPP was lowered from 140 to 40 mmHg in each of the groups. However, coronary venous PO₂ was not associated with changes in coronary blood flow (r = - 0.11; P = 0.29) or autoregulatory gain (r = - 0.29; P = 0.12). Coronary zero-flow pressure (Pzf) was measured in 20 mmHg increments and determined to be directly related to vascular resistance (r = 0.71; P < 0.001). Further analysis demonstrated that changes in coronary blood flow remained minimal at Pzf > 20 mmHg, but progressively increased as Pzf decreased below this threshold value (r = 0.68; P < 0.001). Coronary Pzf was also positively correlated with autoregulatory gain (r = 0.43; P = 0.001). These findings support that coronary autoregulatory behavior is predominantly dependent on an adequate degree of underlying vasomotor tone, independent of normal myocardial oxygen tension.

Regulation of myocardial oxygen delivery in response to graded reductions in hematocrit: role of K+ channels

This study was designed to identify mechanisms responsible for coronary vasodilation in response to progressive decreases in hematocrit. Isovolemic hemodilution was produced in open-chest, anesthetized swine via concurrent removal of 500 ml of arterial blood and the addition of 500 ml of 37 °C saline or synthetic plasma expander (Hespan, 6% hetastarch in 0.9% sodium chloride). Progressive hemodilution with Hespan resulted in an increase in coronary flow from 0.39 ± 0.05 to 1.63 ± 0.16 ml/min/g (P < 0.001) as hematocrit was reduced from 32 ± 1 to 10 ± 1% (P < 0.001). Overall, coronary flow corresponded with the level of myocardial oxygen consumption, was dependent on arterial pressures ≥ ~ 60 mmHg, and occurred with little/no change in coronary venous PO₂. Anemic coronary vasodilation was unaffected by the inhibition of nitric oxide synthase (L-NAME: 25 mg/kg iv; P = 0.92) or voltage-dependent K⁺ (K _V) channels (4-aminopyridine: 0.3 mg/kg iv; P = 0.52). However, administration of the K _ATP channel antagonist (glibenclamide: 3.6 mg/kg iv) resulted in an ~ 40% decrease in coronary blood flow (P < 0.001) as hematocrit was reduced to ~ 10%. These reductions in coronary blood flow corresponded with significant reductions in myocardial oxygen delivery at baseline and throughout isovolemic anemia (P < 0.001). These data indicate that vasodilator factors produced in response to isovolemic hemodilution converge on vascular smooth muscle glibenclamide-sensitive (K _ATP) channels to maintain myocardial oxygen delivery and that this response is not dependent on endothelial-derived nitric oxide production or pathways that mediate dilation via K _V channels.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

The key role of nitric oxide in hypoxia: hypoxic vasodilation and energy supply-demand matching

SIGNIFICANCE

A mismatch between energy supply and demand induces tissue hypoxia with the potential to cause cell death and organ failure. Whenever arterial oxygen concentration is reduced, increases in blood flow--hypoxic vasodilation--occur in an attempt to restore oxygen supply. Nitric oxide (NO) is a major signaling and effector molecule mediating the body's response to hypoxia, given its unique characteristics of vasodilation (improving blood flow and oxygen supply) and modulation of energetic metabolism (reducing oxygen consumption and promoting utilization of alternative pathways).

RECENT ADVANCES

This review covers the role of oxygen in metabolism and responses to hypoxia, the hemodynamic and metabolic effects of NO, and mechanisms underlying the involvement of NO in hypoxic vasodilation. Recent insights into NO metabolism will be discussed, including the role for dietary intake of nitrate, endogenous nitrite (NO₂⁻) reductases, and release of NO from storage pools. The processes through which NO levels are elevated during hypoxia are presented, namely, (i) increased synthesis from NO synthases, increased reduction of NO₂⁻ to NO by heme- or pterin-based enzymes and increased release from NO stores, and (ii) reduced deactivation by mitochondrial cytochrome c oxidase.

CRITICAL ISSUES

Several reviews covered modulation of energetic metabolism by NO, while here we highlight the crucial role NO plays in achieving cardiocirculatory homeostasis during acute hypoxia through both vasodilation and metabolic suppression.

FUTURE DIRECTIONS

We identify a key position for NO in the body's adaptation to an acute energy supply-demand mismatch.

Exogenous intravascular nitric oxide enhances ventricular function after hemodilution with plasma expander

AIMS

This study evaluated the hypothesis that exogenous nitric oxide (NO) supplementation during acute hemodilution with plasma expander (PE) provides beneficial effects on cardiac function.

MAIN METHODS

Acute hemodilution in golden Syrian hamsters was induced by a 40% of blood volume exchange with dextran 70 kDa. Intravascular NO supplementation after hemodilution was accomplished with a NO donor, diethylenetriamine NONOate (DETA NONOate). The test group was treated with DETA NONOate, while the control group received only vehicle. Left ventricular cardiac function was studied using pressure-volume measurements obtained with a miniaturized conductance catheter.

KEY FINDINGS

Cardiac output increased to 122±5% and 107±1% of the baseline in the group treated with NO donor and the vehicle group, respectively. Stroke work per stroke volume (SW/SV) after hemodilution reduced to 90% of the baseline and the NO donor significantly reduced SW/SV compared to the vehicle. The minimum rate of pressure change (dP/dt(min)) was significantly lower in animals treated with the NO donor compared to vehicle treated animals. Systemic vascular resistance (SVR) decreased to 62±5% of the baseline in the NO donor group whereas the vehicle group SVR decreased to 83±5% of the baseline. Using intravital microscopy analysis of microvessel in the dorsal skinfold window chamber, we established that the NO donor group induced significant vasodilation compared to the vehicle group.

SIGNIFICANCE

NO supplementation in an acute hemodilution with PE has beneficial effects on cardiac performance. However, the NO supplementation effects with a NO donor are dose-independent and short-lasting.

The variability of blood pressure due to small changes of hematocrit

The hematocrit (Hct) of awake hamsters was lowered to 90% of baseline by isovolemic hemodilution using hamster plasma to determine the acute effect of small changes in Hct and blood viscosity on systemic hemodynamics. Mean arterial blood pressure increased, reaching a maximum of about 10% above baseline (8.6 +/- 5.5 mmHg) when Hct decreased 8.4 +/- 1.9% (P < 0.005). Cardiac output increased continuously with hemodilution. These conditions were reached at approximately 60 min after exchange transfusion and remained stationary for 1 h. Peripheral vascular resistance was approximately constant up to a decrease of Hct of about 10% and then fell continuously with lowering Hct. Vascular hindrance or vascular resistance independent of blood viscosity increased by about 20% and remained at this level up to an Hct decrease of 20%, indicating that the vasculature constricted with the lowered Hct. The results for the initial 2-h period are opposite but continuous with previous findings with small increases in Hct. In conclusion, limited acute anemic conditions increase mean arterial blood pressure during the initial period of 2 h, an effect that is quantitatively similar but opposite to the acute increase of Hct during the same period.